A Cretaceous seep deposite in Tibet: Diagnostic characteristics and the possible driving force

Abstract:

Microbial oxidation of hydrocarbon released from marine sediments at ocean margin triggers deposite of authigenic carbonates. These carbonates excellently archive ancient seepage activies. Here, we report petrographic, mineralogical and geochemical characteristics of a carbonate deposit discovered in the Cretaceous deep-sea fan section in Tibet, providing diagnostic information on hydrocarbon seepage origin. The carbonates occur as clustered, isolated, chimney-like and stratoid concretions, enclosed in the Cenomanian turbidite siltstone. These concretions consist of carbonate (63.2% on average), clastic quartz and feldspar (23.2% on average), and clay minerals (13.5% on average). They exhibit negative δ¹³C values ranging from -34.1‰ to -12.9‰, indicating a carbon origin from seeping thermogenic methane. A relatively deep origin of thermogenic methane is also supported by the lower ⁸⁷Sr/⁸⁶Sr ratios (from 0.706221 to 0.706808). The carbonates have low δ¹⁸O values varying from -13.1‰ to -2.2‰, most of which indicate diagenetic alteration. The negative linearity between oxygen and carbon isotopic values suggests a diagenetic fluid with relatively high δ¹³C values and high temperature. The shale-normalized REE patterns show varied Ce anomalies. Most samples with no real Ce anomaly illustrate reducing environments. The stratoid concretions and an isolated concretion just above the clustered concretions have real negative Ce anomalies suggesting changeable redox conditions and several sudden changes of flux rate possibly resulted from suspended-reactive cycles of seep activities. Several cycles within short time are likely related with the paroxysmal activities of turbidite sedimentation, which would result in fast loading and finally promote hydrocarbon seeps. There are indications of more widely occurring seep deposits in Cretaceous strata located in Tibet, raising the question if all these hydrocarbon seeps are promoted by regional structure activities.

Aknowlegements: This work was supported by the NSFC (41273041).